The present invention relates to an air/fuel ratio control system for an internal combustion engine, for example, of a motor vehicle, and more specifically to such a control system capable of implementing a feedback air/fuel ratio control over a wide range from a rich side to a lean side.
A conventional air/fuel ratio control system is arranged to perform a feedback control only when the engine is warmed up sufficiently and in a limited engine operating region in which a stoichiometric air/fuel ratio is required, and to perform an open loop control without feedback in a warm up period after cold start or in a high engine load region. Therefore, the control accuracy is low, and the exhaust performance and drivability are poor especially in the warm-up period and in the high load region because the open loop control is unable to compensate for undesired influences of production tolerance, wear and aging of the engine and fuel metering system.
Furthermore, in order to improve the fuel economy by utilizing lean combustion of a lean air/fuel mixture, it is important to perform an accurate feedback air/fuel ratio control on the lean side.
Japanese patent provisional publication No. 60-178942 discloses an improved air/fuel ratio control system which has a so-called wide range air/fuel ratio sensor capable of sensing the air/fuel ratio over the wide range from the rich side to the lean side, and a controller capable of performing a feedback control in which a desired air/fuel ratio is varied from the rich side to the lean side in accordance with engine operating conditions.
In this control system, however, a feedback control constant such as a proportional gain of a proportional control action and an integral gain of an integral control action is held unchanged at a fixed value irrespectively of whether the desired air/fuel ratio is lean or rich, so that an accurate and stable feedback control performance cannot be obtained.